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Author Topic: 40 meter vertical  (Read 2142 times)
KJ6MEV
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Posts: 80




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« on: April 08, 2013, 06:40:52 PM »

built a wire vertical..... a little short, 1.2 to 1 on 7.600mhz. I added a base
coil of just 2 turns of wire of about 4 inches in diameter. Now it's below
1.5 to 1 from 7.150 to 7.425. is it normal for such a small length of wire
to cause this dramatic change of the resonant frequency or is something else
going on here?
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K3VAT
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Posts: 748




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« Reply #1 on: April 08, 2013, 07:08:48 PM »

built a wire vertical..... a little short, 1.2 to 1 on 7.600mhz. I added a base
coil of just 2 turns of wire of about 4 inches in diameter. Now it's below
1.5 to 1 from 7.150 to 7.425. is it normal for such a small length of wire
to cause this dramatic change of the resonant frequency or is something else
going on here?

It's always good to build your own antennas.

A lot depends on the 2nd half of your antenna: the radial system.  I didn't see it described so without know a little bit about the ground radial system, it is hard to accurately answer the question.  That notwithstanding, a few turns of base coil can certainly change the antenna's resonant frequency the amount that you are describing.
GL, 73, Rich, K3VAT
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KJ6MEV
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Posts: 80




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« Reply #2 on: April 08, 2013, 07:29:23 PM »

i have 33 feet of wire running up from the coax wire and 5 radials
that are 1 to 4 feet off the ground...they're between 30 to 35 feet
long....the radials are connected to the braid....no chokes, no baluns.
it's fed with 50 feet of rg8x straight into the radio.
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K3VAT
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Posts: 748




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« Reply #3 on: April 08, 2013, 07:38:55 PM »

i have 33 feet of wire running up from the coax wire and 5 radials
that are 1 to 4 feet off the ground...they're between 30 to 35 feet
long....the radials are connected to the braid....no chokes, no baluns.
it's fed with 50 feet of rg8x straight into the radio.

Thanks for the additional info.  Yes, this will work and the coil size/turns sounds feasible.  Regarding the ground system: if you could manage a few more above-ground radials and tune them properly, then I think that your performance would significantly increase.  While a choke-balun isn't a 100% necessity, they do prevent common-mode currents and in a setup like yours where you have a 'marginal' radial field, it may be a good idea to install one of these at the feed point.  The radial field is the 2nd half of your antenna and one needs to optimize it also.  See the website of N6LF http://www.antennasbyn6lf.com/ to get an idea of how to optimize your elevated radial 40M vertical (you'll need to scroll around as the links aren't necessarily handy).

GL, 73, Rich, K3VAT

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KJ6MEV
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Posts: 80




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« Reply #4 on: April 08, 2013, 08:02:01 PM »

thanks for the info....yes, the bandwidth of the antenna seems to get
better as I add radials.... I started with just 2. I'll definately add a few
more. I'll play with a balun later and do a comparison of balun vs. no balun.
It's kinda fun just running it raw and simple. It's a lot quieter than the g5rv, especially
on 40 meters at night. the g5rv is an s-8 noise level with s-6 signals unreadable. On the vert.
the same signal is an s-5 but the noise is s-1......I get a soft, clear(not strong) readable
signal without all the noise..... a nice, mellow antenna to listen to.
   73 from pietro kj6mev
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KF5KWO
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Posts: 52




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« Reply #5 on: April 10, 2013, 10:12:16 PM »

I love the wire vertical I built as well.  Almost nothing beats building your own antenna.  Thanks for sharing!

73 de Jeff, KF5KWO
Helotes, TX
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K4SAV
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Posts: 1848




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« Reply #6 on: April 11, 2013, 07:26:09 AM »

Glad to hear the vertical is working well for you.  As you are finding out, you learn a lot more when you build your own antennas. 

As for performance, it varies with location, and when compared to a dipole it varies with dipole height, direction, and arrival angle of the signal.  Many people have lots of noise on verticals because they receive the low angle noise signals from a city.  My experience, as yours, is that the vertical usually has a better signal to noise ratio for DX compared to as low dipole, but I don't have city noise. 

The arrival angle of the signal make a big difference.  Most of the vertical's signal is concentrated at low angles while most of the signal from a low dipole are at high angles.  So the dipole may be better for close signals and the vertical better for DX.  If the dipole is very high, then you may find that it is better than the vertical for DX.  Also the direction of signal arrival makes a big difference.  The dipole has lobes and nulls, the vertical doesn't, so the results will depend on which direction the signal comes from. 

These are some of the reasons people say you can never have too many antennas.  You may also find that transmitting on one antenna and listening on the other may provide some advantages.

Jerry, K4SAV
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N3OX
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Posts: 8847


WWW

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« Reply #7 on: April 11, 2013, 10:06:38 AM »

I added a base
coil of just 2 turns of wire of about 4 inches in diameter. Now it's below
1.5 to 1 from 7.150 to 7.425. is it normal for such a small length of wire
to cause this dramatic change of the resonant frequency

Sounds reasonable to me.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
Member

Posts: 13356




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« Reply #8 on: April 11, 2013, 11:21:42 AM »

Quote from: KJ6MEV
built a wire vertical..... a little short, 1.2 to 1 on 7.600mhz. I added a base
coil of just 2 turns of wire of about 4 inches in diameter. Now it's below
1.5 to 1 from 7.150 to 7.425. is it normal for such a small length of wire
to cause this dramatic change of the resonant frequency or is something else
going on here?


Let's see...

A 4" diameter coil contains something over 12" of wire in each turn.  Two turns
adds more than 2 feet.

If we use the traditional formula of 234/f for a quarter wave vertical it gives us
a predicted length of 30' 9.5" at 7.6 MHz and 32' 0.5" at 7.3 MHz (about where
your antenna is now centered).  The difference of 1' 3" is less than the amount
of wire in your coil.

While in general the length of wire used in a coil is not a good indicator of the
electrical lengthening it provides to an antenna, in this case the same "small
length of wire" would shift the resonant frequency down to 7.136 MHz according
to the traditional formula.

So I'd say that sounds reasonable.
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W5WSS
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Posts: 1747




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« Reply #9 on: April 11, 2013, 07:08:21 PM »

Yeah all good. I have a question. Are your radials sloping downward towards the ground from the feed point height of 4ft?
I built a wire one hoist up and held vertical with strain relief and my radials did slope downward from apx 4ft too and the antenna worked really well just curious.
Can you expound on the radial system?
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KJ6MEV
Member

Posts: 80




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« Reply #10 on: April 11, 2013, 07:53:36 PM »

vertical base about 2 feet off the ground. radials now between 1 and 4 feet off the ground, just strung
through shrubs or tied off to stakes.... and yes, the g5rv is better for the short stuff. I'm in calif. and most domestic
stuff,(even florida) is better on the g5rv.... but the vertical is better on the long stuff, like switzerland and russia last night.
the signal might be stronger on the g5rv long stations but I can HEAR the voices better on the vertical because of the lower
noise level, especially later in the evening when 40 meter band conditions change for the worse.... and yes I plan on adding
more radials soon.
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KJ6MEV
Member

Posts: 80




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« Reply #11 on: April 11, 2013, 07:57:52 PM »

and thanks wb6byu for the math.... I've been reading EASY UP ANTENNAS by Ed Noll.
this is where I'm getting a lot of the info on wire lengths and such. I recommend this old-
school book from yesteryear.
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W5WSS
Member

Posts: 1747




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« Reply #12 on: April 11, 2013, 08:32:23 PM »

Yes your elevated sloped radials appear to be isolated from earthing and this is a very good method too often misunderstood.
The isolation from earth adds system balance because the antenna is behaving as balanced antenna rather than an end fed with an earth connection ground rod very noisy technique.

For an antenna lightning safety technique you could add a ground rod located underneath the antenna or nearby and attach a short length wander lead (not to be connected in series with the antenna radials during use) but only connected in series with the antenna base wire for a diversionary path to earth outside the shack when thunderstorm potential and off the air or away.

One can permanently install the station to the ac electrical mains panel ground rod for the station safety ground.
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